1. Field of the Invention
The present invention relates to a color cathode ray tube apparatus and, more particularly, to a general color cathode ray tube having high image quality such as an EDTV or HDTV.
2. Description of the Related Art
A general color cathode ray tube apparatus having high image quality comprises a tube provided with a panel, a funnel contiguous with the panel, and a cylindrical neck connected to the funnel. A shadow mask is arranged inside the panel, and a phosphor screen surface comprising a tri-color light emitting layer is formed on the inner surface of the panel to oppose the shadow mask. A large number of apertures are formed in the shadow mask. The shadow mask has a frame on its periphery, and is supported on the panel through the frame. An internal magnetic shield is mounted on the frame. An internal conductive film is coated from the inner wall of the funnel to a portion of the neck. An external conductive film is coated on the outer wall of the funnel, and an anode electrode is provided to a portion of the funnel. An electron gun for outputting three electron beams is accommodated in the neck. A deflection device is arranged outside a boundary portion between a cone portion of the funnel and the neck so as to deflect three electron beams emerging from the electron gun in horizontal and vertical directions. In addition, a driver for applying an appropriate voltage to the electron gun and the anode electrode and supplying a voltage to the deflection device is arranged.
Red, green, and blue phosphor stripes or dots are distributed and coated on the phosphor screen surface. Three electron beams Br, Bg, and Bb emerging from the electron gun toward the phosphor screen surface are deflected by the deflection device. The electron beams Br, Bg, and Bb are selected by the shadow mask, and then become incident on the phosphor screen. Thus, the corresponding phosphors emit light to form an image. In an electron gun having an in-line arrangement, three parallel electron beams are generated. This electron gun has an electron beam forming unit GE for generating, controlling, and accelerating three electron beams, and a main electron lens unit ML for focusing and converging these electron beams.
A deflection yoke as the deflection device has horizontal and vertical deflection coils for deflecting the three electron beams in the horizontal and vertical directions. In the deflection yoke for deflecting inline aligned electron beams, in order to precisely converge electron beams, a horizontal deflection magnetic field is formed into a pin-cushion pattern, and a vertical deflection magnetic field is formed into a barrel pattern, thus constituting a so-called convergence free system.
A general color image pickup tube is required to have a small depth, low power consumption, and high resolution over the entire screen. However, these requirements confront technical limitations, and pose very difficult problems. These problems will be briefly summarized below.
(1) Small Depth Requirement
In order to realize this, a deflection angle of electron beams is increased. However, when the deflection angle of the electron beams is increased, a deflection current is increased, and power consumption is also increased. Furthermore, deflection defocusing and a difference between moving distances of electron beams are increased, thus impairing both convergence and focusing.
(2) Low Power Consumption Requirement
In order to achieve this, a neck diameter can be decreased to increase a deflection sensitivity, and a deflection angle can be decreased. However, when the neck diameter is decreased, focusing is impaired to decrease a resolution. Furthermore, when the deflection angle is decreased, this inevitably leads to an increase in depth.
(3) High Resolution Requirement Over Entire Surface
In order to achieve this, a deflection angle can be decreased, and a correction coil and a digital convergence circuit can be added. However, when the deflection angle is decreased, this inevitably causes an increase in depth. Furthermore, new circuits, in particular, the digital convergence circuit requires large power consumption, thus increasing power consumption as a whole.
The above-mentioned problems will be described in detail below.
In order to decrease a depth of a deflection of electron beam, a maximum deflection angle of electron beams deflected by the deflection yoke can be increased. However, when the deflection angle is increased, a deflection current flowing through the deflection coils is increased, resulting in an increase in power consumption. In order to reduce power consumption, the neck diameter can be decreased to increase a deflection sensitivity. However, when the neck diameter is decreased, the aperture of an electron lens of the electron gun is inevitably decreased, and two side electron beams tend to be easily influenced by an aberration of the electron lens, thus increasing a beam spot size on the screen. As a result, resolution is decreased. Furthermore, in the problem of power consumption, an electrical power supplied to the horizontal deflection coil particularly poses a problem. This problem is posed in the NTSC method since a horizontal deflection frequency (15,750 Hz) is much higher than a vertical deflection frequency (60 Hz) (i.e., about 260 times). When an impedance of the horizontal deflection coil is represented by L.sub.H (mH), and a current is represented by i" (A), power consumption is expressed by L.sub.H .multidot.(i.sub.H.sup.2) mH.multidot.(A.sup.2). When power consumption is large, this poses not only an economic problem but also a fatal problem such that the deflection yoke is heated and burnt. The critical temperature of the deflection yoke is 60.degree. C. according to its constituting material.
When the deflection angle is increased, another problem is posed. That is, when the deflection angle is large, a difference between flying distances of electron beams on the central portion and the peripheral portion of the screen becomes very large, resulting in poor focusing of electron beams by the electron gun. Furthermore, since deflection defocusing caused by the deflection yoke is increased, resolution is considerably decreased on the peripheral portion of the screen. In order to decrease the spot size on the screen, the neck diameter must be increased to increase the electron lens aperture of the electron gun. However, since three electron lenses are linearly aligned, the diameter of the electron gun is increased. Thus, a deflection sensitivity is impaired, and it is difficult to attain good convergence of the three electron beams on the entire screen. As a result, resolution and sharpness are impaired.
A home color cathode ray tube will be exemplified below. For example, a screen diagonal dimension is 32"; a deflection angle, 110.degree.; a depth, about 500 mm; a neck inner diameter, 26.0 mm; a neck outer diameter, 32.5 mm; a lens aperture (beam passage hole diameter) of an electron gun, 6.2 mm; an interval of in-line aligned three electron beams, 6.6 mm; a length of the deflection yoke along the tube axis, 75 mm; an opening on the electron gun side of the yoke, 35 mm; and an opening on the screen side of the yoke, about 140 mm. The deflection yoke has saddle-type horizontal and vertical deflection coils each of which is formed by winding a single wire. A spot size of the electron beams on the screen is about 2 mm when the current value of the electron gun is 1 mA. A consumption current L.sub.H .multidot.(i.sub.H.sup.2) of the coil is about 42 mH.multidot.(A.sup.2) (anode voltage =32 kV). When deflection is performed at a horizontal deflection frequency of 15.75 kHz and a vertical deflection frequency of 60 Hz, heat generation is about 35.degree. C. In addition, convergence quality is about 2.0 mm on the peripheral portion of the screen.
A color cathode ray tube used in a television system such as an EDTV or HDTV is required to have higher image quality than the above-mentioned cathode ray tube. However, if quality is improved in a video signal system, various problems of the color cathode ray tube as a whole are posed, and it is very difficult to improve image quality.
Since the HDTV is required to have very high quality, various color cathode ray tube apparatuses have been manufactured as samples. However, these apparatuses are very disadvantageous as home color cathode ray tubes as follows.
For example, in a color cathode ray tube apparatus having a screen diagonal dimension of 32", a deflection angle of electron beams is 90.degree., and a depth of the tube is about 660 mm. Thus, the depth is larger than a conventional tube apparatus by 160 mm. For this reason, such a tube apparatus is too large for a home use, resulting in large industrial and economic losses.
In this tube, the neck has an inner diameter of 30.9 mm and an outer diameter of 36.6 to 37.5 mm. Three electron beams of the electron gun are delta-aligned, and the aperture of one electron lens (beam passage hole diameter) is 12.0 mm. The aperture of one electron lens is about twice that of a general home tube apparatus. Since a resolution of 1,000 (TV) lines is required, a beam spot size on the screen is about 1.2 mm (electron gun current value Ik =1 mA), i.e., decreased by about 40% as compared to that of the home tube apparatus. When the electron lens aperture is increased, the spot size of the electron beams is decreased accordingly. Therefore, when the electron lens becomes large in size, the spot size on the screen can be decreased. That is, when the lens aperture is determined, an electron optical magnification is determined. The same applies to other types of electron lens (e.g., bipotential type, unipotential type). In this apparatus in the HDTV required to have a resolution as high as 1,000 (TV) lines, a lens aperture of about 12 mm or more is required. However, when three electron beams are inline aligned in an electron gun having a neck inner diameter of 30.9 mm, the aperture of one lens is a maximum of 9 mm, and it is impossible to increase the aperture to 12 mm or more. Since three electron beams are delta-aligned, good convergence cannot be obtained over the entire screen by the above-mentioned convergence free magnetic field distribution. Therefore, a new convergence correction coil must be added, resulting in large industrial and economic losses, and an expensive color cathode ray tube apparatus.
Furthermore, the HDTV is required to have a maximum miss-convergence amount of 0.3 to 0.5 mm (about 0.1% or less of a screen height). However, such high-precision convergence cannot be obtained by only the above-mentioned correction coil. For this reason, a digital convergence circuit is added. Since this digital convergence circuit is expensive and requires a high electrical power, it is not suitable for a home use. If convergence is set using the digital convergence circuit, it must be set and stored at several tens of positions on the entire screen one by one. For this reason, much time is required in the manufacture. Therefore, the digital convergence circuit cannot be used in general color cathode ray tube apparatuses which must be mass-produced. In addition, industrial and economic losses are large, and cost becomes several to several tens of times that of existing home color cathode ray tube apparatuses.
Power consumption L.sub.H .multidot.(i.sub.H.sup.2) of the deflection yoke for deflecting electron beams through 90.degree. by generating identical magnetic fields from its saddle-type horizontal and vertical deflection coils is about 35 mH.multidot.(A.sup.2) and is lower than that required when beams are deflected through 110.degree.. Therefore, no heat problem caused by heat generation occurs. However, when the deflection angle of electron beams is increased to be larger than 90.degree., power consumption is abruptly increased, and a problem of heat generation is posed accordingly. In addition, convergence is impaired. When the electron beams are deflected through a wide angle of 100.degree. or more, a spot of the electron beams causes a considerable halo on the peripheral portion of the screen due to deflection defocusing by the deflection yoke. As a result, resolution is considerably decreased. The above-mentioned delta-aligned electron gun cannot improve such deflection defocusing as a dynamic focus.
As described above, a television system is required to provide a high-quality image. However, a color cathode ray tube apparatus having a high-quality image poses problems of a large tube depth, high power consumption, and very high cost.